Because a Multiple Input Multiple Output (MIMO) technology plays an important role in a peak rate and a system spectrum utilization rate, such wireless access standards as Long Term Evolution (LTE) and LTE-Advanced (LTE-A) are constructed on the basis of a MIMO+Orthogonal Frequency Division Multiplexing (OFDM) technology. A performance gain of the MIMO technology is derived from a spatial freedom degree capable of being achieved by a multi-antenna system, so during the standardization of the MIMO technology, the most important evolution direction lies in the extension of dimensions. In LTE Release 8, at most four MIMO transmission layers may be supported. In LTE Release 9, a Multiple-User (MU)-MIMO technology has been enhanced, and at most four downlink data layers may be supported by the MU-MIMO with a Transmission Mode (TM)-8. In LTE Release 10, a spatial resolution of Channel State Information (CSI) has been further improved by introducing an 8-port CSI Reference Signal (CSI-RS), and Demodulation Reference Symbol (DMRS) and a multi-granularity codebook, and the transmission capability of Single-User MIMO (SU-MIMO) has been extended to at most eight data layers.
In addition, along with the maturity of an Active Antenna System (ASS) technology and the application of an AAS array in a two-dimensional (2D) plane, the MIMO technology is moving in a three-dimensional (3D) and massive direction. Currently, the 3rd Generation Partnership Project (3GPP) is studying 3D channel modeling, and in future, it is expected to study and standardize an Elevation Beamforming (EBF) technology using 8 or fewer ports, and a Full Dimension MIMO (FD-MIMO) technology using more than 8 ports (e.g., 16, 32 or 64 ports). In academia, the MIMO technology on the basis of a massive antenna array (including a hundred of, or hundreds of, or more antenna elements) is now being studied and tested proactively. The research and the preliminary channel test result show that, a massive MIMO technology can improve the system spectrum efficiency remarkably and support more users to access. Hence, the massive MIMO technology has been considered by various research organizations as one of the most potential physical layer technologies for a next-generation mobile communication system.
For the MIMO technology, particularly the MU-MIMO technology, the precision of the CSI capable of being acquired at a network side directly determines the precision of precoding/beamforming and the computational efficiency of a scheduling algorithm, and thereby affecting the performance of an entire system. Hence, the acquisition of the CSI is always one of the core issues in the standardization of the MIMO technology. For a Frequency Division Duplex (FDD) system, there is a relatively large frequency interval between an uplink and a downlink, and usually it is very difficult to directly acquire downlink CSI by measuring an uplink channel. Hence, a CSI measurement and feedback mechanism on the basis of a downlink reference signal is usually adopted by the conventional FDD system. In this case, the spatial resolution of the CSI directly depends on the number of ports for the reference signals. In the case of a very large antenna array, new ports for the reference signals may be introduced so as to ensure the downlink transmission, but this will lead to obvious time-frequency resource overhead. However, in the case that the number of the ports for the reference signals is limited, it is impossible to ensure the spatial resolution for the measurement of the downlink CSI, and as a result, it is impossible for the Massive MIMO technology to play to its strengths.
In a word, on the premise that the Massive MIMO technology plays to its strengths, the CSI measurement and feedback mechanism on the basis of the downlink reference signal may lead to the obvious time-frequency resource overhead.